Fluid velocity transducer

ABSTRACT

A transducer for measuring the relative velocity of movement of a body in a liquid or gaseous medium employs no reciprocating parts and functions by sensing the force applied to a diaphragm ( 6 ) die to its movement through the medium or the movement of the medium towards the diaphragm. The speed transducer makes use of a Hall effect solid state device ( 7 ) that produces an output voltage when a magnetic field is applied to either of its surfaces. The level of the output voltage is proportional to the velocity of movement in the positive direction, i.e. when the force acts on diaphragm ( 6 ) in (FIG.  1 ). Since the transducer takes into account differential forces, an unwanted common force acting on both diaphragms simultaneously will have no effect on the measured speed. The tubular body ( 1 ) in which the Hall effect device is located may be made from any non-magnetic material suitable for the environment in which the transducer will be used.

[0001] This invention relates to a velocity measuring device and isconcerned with a transducer for measuring the relative velocity ofmovement of a fluid medium, for example water or air, either when thetransducer is made to move through the fluid medium or the fluid mediummoves towards the stationary transducer.

[0002] Many velocity transducers are in common use but most requirereciprocating parts that wear or become stuck for one reason or another.For example, when used in a marine environment, the paddle wheel typemust be serviced regularly because it is affected by weeds, barnaclesand mud. For air or gas velocity measurement, the bearings of therotating part must be of sufficient high quality to provide a reasonablylong life and hence long-term, reliability.

[0003] It is an object of this invention to provide a velocitytransducer that does not rely on reciprocating parts or bearings andwill not require regular servicing even in a marine environment.

[0004] According to the invention, there is provided a fluid velocitytransducer comprising a substantially tubular body having a fluid-tightoutlet for signal wires, the body having located therein a Hall effectunit, and a pair of oppositely facing diaphragms positioned respectivelyat each end portion of the tubular body, each diaphragm having a magnetfixed to its interior surface, the arrangement being such that in useone of the diaphragms can receive an applied force due to a fluid andconvert this force into a displacement relative to the Hall effect unitthereby increasing the magnetic field strength applied to the Halleffect unit so as to produce a signal output which can be processed toprovide an indication of the speed of movement.

[0005] The transducer of the invention measures velocity by making useof a very small displacement produced by the force of the medium inwhich it moves or the force of the medium that moves on to it. Thetransducer responds to the differential forces applied to its surfacesand produces a signal output only when the two forces applied areunequal. The output signal is proportional to the velocity in thedirection of motion and very little or no output is produced in otherdirections, except in the reverse direction if the transducer or mediumis reversed.

[0006] The tubular body of the transducer may be made from any materialsuitable for the environment in which it is to be used. For a marineapplication, a plastics material such as, for example an ABS(acrylonitrile-butadiene-styrene) copolymer, is suitable for fabricationof the tubular body and also of the diaphragms. The Hall effect unit isadvantageously mounted on a printed circuit board positioned in acentral region of the tubular body. The diaphragms can be fixedlyattached to the respective ends of the tubular body, for example bychemical bonding or thermal welding, so as to provide a fluid-tight sealfor each end of the tubular body. The magnets attached to each diaphragmrespectively are preferably of different sizes.

[0007] The velocity transducer of the invention can be used to measurethe speed of movement of a vessel, for example a ship or boat, throughwater, for which purpose it will be fixed to the hull of the vesselbelow the waterline. Other uses may be to measure the speed of waterflow in a river or the flow rate of a liquid or gas through a pipeline.It may also be used to measure wind speed on land or the sea.

[0008] The signal output from the Hall effect unit may be coupled to aninterface unit to process the varying direct current signal from thetransducer for digital display.

[0009] If desired, a thermistor may be attached to the printed circuitboard in addition to the Hall effect unit to enable the ambienttemperature of the liquid or gas to be measured.

[0010] For a better understanding of the invention and to show how thesame may be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings in which:

[0011]FIG. 1 is an enlarged diagrammatic longitudinal axialcross-section of a velocity transducer; and

[0012]FIG. 2 is a perspective view of the velocity transducer of FIG. 1.

[0013] Referring to the drawings, a liquid speed transducer has a bodycomprising a length of ABS plastics tubing 1. A watertight outlet 4,advantageously positioned in alignment with a central region of thetubular body 1, provides a path for electrical wires (not shown) for anoutput signal.

[0014] A linear Hall effect unity 7 is mounted in the central region ofthe tubular body on a printed circuit board 8.

[0015] A first diaphragm 3 and a second diaphragm 6 can transmit smallmovements from the outside and at the same time serve to seal the unit.The diaphragms both made of ABS plastics material, are fixedlypositioned in recessed rims formed respectively at each end of thetubular body 1. The diaphragms have substantially equal surface areas.

[0016] A small magnet 5 is mounted on the central area of diaphragm 6for which a suitable recess is provided. Likewise a larger magnet 2 ismounted on diaphragm 3 and fixed in a recess provided therefor.

[0017] A magnetic field from magnet 5 acting on the Hall effect unit 7causes it to produce a differential output voltage. A magnetic fieldfrom magnet 2 acting on the Hall effect unit on the reverse side reducesthe output voltage. The two magnets are placed relative to the Halleffect unit so that the output is zero with no external forces applied.The magnetic fields are in opposition to each other.

[0018] An external force acting on diaphragm 6 increases the magneticfield strength on the Hall effect unit 7 by reducing the gap betweenmagnet 5 and unit 7 thus producing a positive output voltageproportional to the displacement of diaphragm 6 as a result of theapplied force. Conversely external force acting on the diaphragm 3reduces the output voltage.

[0019] The transducer will detect the velocity of liquid flowing on todiaphragm 6 because the displacement of the diaphragm produces aproportional output voltage as a result of the force of the liquidacting on the diaphragm. The unit will detect its own velocity only ifit is moved through liquid in a direction so that the applied force actspreferably on diaphragm 6. Force acting on diaphragm 3 at the same timewill have a negative effect and hence reduce the output, enabling thetransducer to compensate for movement in other directions.

1. A fluid velocity transducer comprising a substantially tubular body(1) having a fluid-fight outlet (4) for signal wires, the body havinglocated therein a Hall effect unit (7), and a pair of oppositely facingdiaphragms (3,6) positioned respectively at each end portion of thetubular body (1), each diaphragm having a magnet (2,5) fixed to itsinterior surface, the arrangement being such that in use one of thediaphragms can receive an applied force due to a fluid and convert thisforce into a displacement relative to the Hall effect unit therebyincreasing the magnetic field strength applied to the Hall effect unit(7) so as to produce a signal output which can be processed to providean indication of the speed of movement.
 2. A fluid velocity transduceras claimed in claim 1, wherein the fluid-tight outlet (4) for signalwires is positioned in alignment with a central region of the tubularbody (1).
 3. A fluid velocity transducer as claimed in claim 1 or 2,wherein the Hall effect unit (7) is mounted on a printed circuit board(8) in a central region of the tubular body (1).
 4. A fluid velocitytransducer as claimed in claim 1 or 2, wherein the diaphragms (3,6) arefitted with magnets (2,5) of different sizes.
 5. A fluid velocitytransducer as claimed in claim 4, wherein the respective spaces betweenthe diaphragms (3,6) and the surfaces of the Hall effect unit (7) aresuch as to produce zero output voltage when no external forces areapplied.
 6. A fluid velocity transducer as claimed in claim 1 or 5,wherein the two diaphragms (3,6) have substantially equal areas.